Materials which diameters are less than 100 nm (it paraphrases it in this description like this from now on; “nano-sized substance”) are known to have a large specific surface area and to exhibit different characteristics from those of substances which are generally known.
In particular, metals are found to exhibit phenomena, because of reactivity increases at the surface of the fine particle, melting point is decreased.
Because of their unique characteristics, nano-sized substance is promising at the field of electronic devices; the production of equipments to small size, and the provision at low cost.
Downsizing of electronic devices are mainly blessed with improvements in the degree of integration of semiconductors.
Furthermore, higher integration of the pattern connecting between the semiconductors on a printed substrate is also necessary for the downsizing of electronic devices.
It is desirable for such a highly integrated pattern to have a narrow line width and a high electrical conductivity.
It was able to overcome such demands; they were able to use some methods, for example, plating, vapor deposition and sputtering.
However, we need to prepare big apparatus when we'll use those methods. And we have to renew when we need to more fine patterns, so they aren't suitable for drawing of finer lines.
Thus, if we can get apparatus which has compact and high efficiency, it's very comfortable to draw of fine metallic lines.
Examples of methods for forming a drawing pattern which satisfy these requirements include methods employing a printing technique, such as inkjet printing and screen printing.
For example, to form a drawing pattern on the substrate by a printing technique, a conductive paste in which a fine metal powder is dispersed in a binder is used.
However, problems with this method are that the precision of the pattern formation is low, and that the electrical conductivity of the produced pattern is lower than that of the original metal.
Conventional conductive pastes, which dispersed in an organic resin binder, contains of metal particles which average diameter is from several tens to hundreds microns. Thus, it is difficult to form fine line more than a diameter of average particle size.
Furthermore, the printed pattern has a structure in which the metal powder contacts each other at points. Consequently, the electrical conductivity is substantially worse than that of a sheet shape of the pure metal.
One way to resolve such problems of a conductive film formed by printing is to use the above-described nano-sized metal particles.
If such fine metal particles are used, a high-precision drawing pattern can be formed at a higher density than conventional paste. Consequently, we can get increasing the number of particles per unit volume; so electrical conductivity can also be expected to improve to closer to that of a sheet shape of the pure metal.
In addition, because of increases of the fine particle reactivity, melting point is decreased. Because of those phenomena, we can get a sheet of metallic at lower sintering temperature than conventional.
From this perspective, various methods have been investigated which was using nano-sized metal particles, and several proposals have already been made.
As methods for producing nano-sized metal particles, gas-phase methods and fluid-phase methods are mainly known.
For example, Patent Document 1 describes that a dispersion of independent ultra fine silver particles can be obtained by producing ultra fine particles of silver by a gas-phase method in vacuo and then mixing these particles with an organic solvent. In this dispersion, the surface of the ultra fine particles is covered with the organic solvent, so that each of the particles is independently dispersed.
Patent Document 2 describes ultra fine particles obtained by a fluid-phase method.
In this technique, metal fine particles formed by reduction of metal ions in an aqueous phase undergo a phase transfer from the aqueous phase to a more stable organic solvent phase.
More specifically, a small amount of a protective colloid is made to be present in the organic solvent in advance so that the metal fine particles undergo a phase transfer from the aqueous phase to the stable organic solvent layer, and are formed as stable colloid particles. This allows the metal particles to be obtained in a high concentration in the organic solvent phase.
Patent Document 3 also describes a production method in the fluid phase.
Patent Document 3 describes that during the production of silver nano-particles by reducing a silver salt in a solvent, rather than the typically used silver nitrate, a silver halide (especially silver chloride or silver bromide), which is insoluble as a silver salt, is used. Furthermore, in the method described in Patent Document 3, the reduction is carried out in the presence of a protection agent formed from a compound which is dissolved in the solvent and which can be coordinated with silver.
In this method, it is described that a mono-dispersion can be obtained in which silver nano-particles are coated/protected by the protection agent and dispersed in the solvent.
It is also described that a polar solvent is used as the solvent, and a thiol, such as thiocholine bromide, is preferred as the protection agent.
Patent Document 4 also describes a method for obtaining nano-order fine silver particles which are mono-dispersed in a polar solvent using a fluid-phase method.
This document describes that silver nitrate is used as the starting material for obtaining the fine particles of silver, and that heptanoic acid is used as the protection agent.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-35255
[Patent Document 2] Japanese Patent Application Laid-Open No. Hei. 11-319538
[Patent Document 3] Japanese Patent Application Laid-Open No. 2003-253311
[Patent Document 4] US 2007/0144305 A1